Sealed off radio-frequency (RF) excited carbon dioxide (CO2) lasers are currently favored for drilling via-holes (via-drilling) in PCBs. These lasers are relatively compact in relation to available output power. By way of example, a laser less than one meter (m) in length can deliver a beam of long-wavelength infrared (IR) radiation at wavelength around 10.6 micrometers (μm) with an average power of 400 Watts (W) or greater. While CO2 laser via-drilling is fast and efficient, there will be an eventual limit to the smallest hole-size that can be drilled due to the long wavelength of the laser beam. Demands on minimum hole-size and spacing are increasing rapidly for PC boards used in so called “smart phones”, which, practically considered, are hand-held portable computer devices with many more functions than simply making and receiving phone calls. In a current state-of the-art smart phone, a PCB may have as many as 30,000 via-holes. As more functions are added to the smart phones, more complex circuitry with more via holes will be required, and, eventually a shorter drilling wavelength will be required to achieve smaller hole-size and closer spacing.
It has been postulated that a preferred via-hole drilling wavelength would be a so-called mid-IR (MIR) wavelength between about 2 μm and about 8 μm. Apart from the potential for smaller hole-size and closer spacing, MIR wavelengths have a higher absorption coefficient in PCB materials than at the longer, CO2 laser radiation wavelengths. This would allow a more rapid absorption of heat into the PCB, which could lead to holes with cleaner walls and less collateral thermal damage in general.
MIR wavelengths could be generated from commercially available near-IR (NIR) emitting, solid-state lasers, fiber-lasers, or optically pumped semiconductor lasers, using sum-frequency generation or optical-parametric generation. This however would require apparatus having many times the cost-per-watt achievable in a CO2 laser system.
Arguably, the only potentially viable candidate MIR laser for replacing a CO2 laser for via-drilling is a CO laser. Recent investigations of sealed off CO-lasers have led to a sealed off CO-laser which is, with only a simple modification, a sealed-off CO2 laser but with a different lasing-gas mixture. A power output of about 80% of that of a corresponding CO2 laser has been achieved.
In via-drilling operations, CO2 lasers are driven in a pulsed mode. A problem with CO lasers is that when driven in a pulsed mode, pulse rise and fall times are relatively long. Long rise and fall times of pulses can create unacceptable collateral thermal damage around laser-drilled via holes. In theory at least, rise and fall times of laser pulses can be shortened by modulation “clipping” of the laser-pulses using an acousto-optic modulator (AOM).
This is complicated, however, by a broad wavelength range of CO laser output. CO laser output occurs at range of laser wavelengths between about 4.5 μm and about 6.0 μm. An AOM functions by virtue of a refractive index grating induced in a susceptible crystal such as a germanium (Ge) crystal by applying a high RF voltage to the crystal. Inducing the grating diverts a laser beam from one path through the crystal with no grating induced, into an alternate path at an angle to the un-diverted (RF applied) path. The angle of the diverted path, of course, is wavelength dependent. With a collimated beam having the full bandwidth of the CO laser, a diverted beam would be spread into a fan of rays which would complicate focusing the beam onto a PCB for drilling.
This beam-spreading by an AOM could be mitigated by limiting the CO laser bandwidth, for example, by using a spectrally selective device such as an etalon or grating within the laser-resonator. The nature of the CO laser, however, is such that output power would be reduced in direct proportion to the degree of spectral-bandwidth reduction. Reducing the spectral bandwidth to proportions compatible with the AOM could reduce the CO laser power to as low as one fifth of that of a comparably sized and pumped CO2 laser. This and related problems must be overcome for a CO laser to become a commercially viable replacement for a CO2 laser for via-drilling.